Method of foaming granulated metal



March 7, 1961 S. O. FIEDLER EAL METHOD OF FOAMING GRANULATED METAL Filed Dec. 12, 1957 INVENTORS JOHA/V BJORKSTEN STUARTO. F/EDLE'R WILLIAM .S. F/EDLER Art'arney METHOD OF FOAMING GRANULATED METAL Stuart 0. Fiedler, Johan A. Bjorksten, and William S. Fiedler, Madison, Wis., assignors, by direct and mesne assignments, to Lor Corporation, a corporation of Delaware Filed Dec. 12, 1957, Ser. No. 702,745

21 Claims. (Cl. 75-20) This invention relates to the manufacture of metal foam and in particular, to a method for making metal foam by foaming granulated metal.

This is a continuation-in-part of our copending application Serial No. 655,936, filed April 8, 1957, now abandoned, having the same title.

A metal foam has been desired having the properties of low weight per unit volume and exceptional rigidity and toughness and capable of being formed in a mold or in specific shapes.

Heretofore, a variety of methods have been used for making metal foams. However, no method has been found which allows the foaming of metal in one step and allows the manufacture of metal foam having specific shapes.

One method which has been proposed for making foamed aluminum is the vaporization of mercury in aluminum. This method is highly unsatisfactory as the materials utilized have a deleterious effect on each other and the amount of gas evolved in the reaction is difficult to control. The reaction must be carried out in a pressurized chamber; high pressures being exerted on a mixture of metals to build up a high vapor pressure of the volatile metal and then lowering of the pressure to cause foaming.

Another method proposed for the manufacture of metal foam. comprises the addition of an alloy comprising ground metal hydride directly to the surface of the metal matrix; the alloy being prepared for addition by grinding together metal hydride and one or more molten metals and then cooling and reducing the resultant product to a powder. In this process, it is necessary to form the hydride mixture by the aforementioned method, the metal to be foamed must then be melted and then the hydride mixture added to the surface of.

the metal matrix. The addition of the hydride creates a difiiculty due to the tendency of the powder to float to the top of the metal matrix and oxidize.

These and other methods involve the use of several involved steps before the final end product, metal foam, is obtained. The present invention proposes the manufacture of metal foam in essentially one step.

It is an object of the present invention to produce a metal foam.

Another object is to produce a metal foam from granulated metal and a material adapted to efiect foaming.

Another object is the production of molded metal foam.

Another object is a method for producing metal foam.

Another object is a continuous method for producing metal foam.

Another object of this invention is to simplify the manufacture of metal foam.

Another object is the manufacture of metal foam involving essentially only one heat utilization step.

Further objects will become apparent from the drawings and the following detailed description in which it is our intention to illustrate the applicability of the invention without thereby limiting its scope to less than that of all those equivalents which will be apparent to one skilled in the art.

In the drawings like reference numerals refer to like parts and:

Figure 1 is a cut-away perspective view of one embodiment of the invention showing a continuous process;

Figure 2 is a cut-away perspective view of another embodiment of the invention showing a continuous process;

Figure 3 is a cut-away perspective view of an embodiment of the invention showing a batch process;

Figure 4 is a cut-away perspective view of an embodiment of the invention showing a batch process; and

Figure 5 is a cut-away perspective view of an embodiment of the invention showing a batch process.

Referring now to the figures, the particles of the invention may comprise either a mixture of metal to be foamed and material adapted to effect foaming or particles prepared by the process of mixing hydride-corn,

taining metal alloy powder into molten metal, cooling said mixture immediately to prevent foaming and grind ing said mixture to form the particles.

Referring now to Figure 1, an apparatus for continuously producing metal foam is given the general designation 1. The apparatus comprises a hopper 2 for holding the particles 3, a trough 10 for carrying the particles from the hopper to the conveyor belt 5, a means 4 for vibrating said trough, a conveyor belt for carrying the particles through the heating zone 6 comprising gas burners'(not shown) and through a cooling zone comprising, in this case, water faucets 11 and 12.

Particles 3 are continuously added to hopper 2. The particles are fed through opening 9 to the vibrating trough 10, activated by means 4 (not shown). The particles vibrate down the trough to the conveyor belt 5 where the particles are uniformly spread thereon. The conveyor belt carries the particles through the heating zone 6 comprising gas burners (not shown) where first the metal melts and the material adapted to effect foaming decomposes to liberate a gas and cause said molten metal to foam. The conveyor belt then carries the molten metal foam through a cooling zone produced by the spray of water from water faucets 11 and 12 to obtain a continuous elongated member of metal foam 13.

Referring now to Figure 2, an apparatus for continuously producing metal foam is given the general designation 14. The apparatus comprises a pressure container 15 having a removable hatch 16 on the top thereof for the introduction of the particles 17 to a vibrating hop- I per and containing within said pressure container a vibrating hopper 18, a means 19 (not shown) for causing said hopper to vibrate, a trough 20 for carrying said particles from the hopper to conveyor belt 21, a conveyor belt for carrying the particles through a heating zone 22 comprising gas burners (not shown), thus causing the metal to melt and the material adapted to-' effect foaming to decompose. The molten mixture is then expelled from the pressure container through an orifice 24 and flows onto another conveyor belt 27 and there the metal foam 26 is air cooled by means of fans 28 and 7. The pressure desired within the pressure con-- tainer may be obtained by introducing a gas into said container from a gas source by means of tube 23; said are mixed down the trough by means of vibration onto conveyor belt 21; said vibrations causing the uniform dispersion of particles thereon. Conveyor belt 21 carries the particles through heating zone 22 created by gas burners. The heat causes the metal to melt and the material adapted to effect foaming to decompose thus building up the vapor pressure within the pressure container sumciently high so that when the molten metal is expelled from the pressure container through orifice 24 the change from high pressure within the container to atmospheric pressure outside of the container is sumcient to cause the gas contained within said molten metal to expand thus producing the metal foam.

The mixture of metal and decomposed foamer is expelled from the pressure container through the orifice to form metal foam and thence flows onto another conveyor where the molten metal foam is cooled by means of fans 28 and 7.

Referring now to Figures 3, 4 and 5, an apparatus for producing metal foam according to a batch process is given the general designation 32.

As shown in Figure 3, particles 31 are uniformly spread on the base of a preheated open mold 32 until suificient particles are present to completely fill the mold on exposure to heat.

As shown in Figure 4, the open mold 32 and particles 31 of Figure 3 are subjected to suflicient heat which is produced by gas burners 33, 34 35 and 36 located above the open mold and gas burners 37, 38, 39 and 40 located below the mold and directed on the base of the mold, to cause the metal to melt and then to cause the material adapted to effect foaming to decompose thus forming the metal foam body 41; said heating to be continued until the foaming agent is completely exhausted.

As shown in Figure 5, the open mold 32 and metal foam body 41 of Figure 4 is immediately cooled, after foaming is completed, by means of fans 42, 43 and 44, thus producing a metal foam body, mold 32 may then be removed from around metal foam body 41 or the mold may be a part of the embodiment.

By metal foam, we mean a body consisting of gascontaining discrete cells distributed in a metal matrix in a generally uniform manner, each cell being entirely enclosed and generally being not connected to any neighboring cell. Metal foam should not be confused with metal sponge which consists of interconnected cells or a plurality of passages as interstices which communicate in a metal matrix. By hollow article, we mean a mold or a hollow pipe or other suitable hollow object which could be suitably filled with metal foam.

The present invention contemplates the manufacture of metal foam by causing decomposition of particles which comprise either a mixture of metal to be foamed and material adapted to effect foaming such as hydride-containing metal alloy or particles which are obtained by first mixing hydride-containing metal alloy with molten metal, then rapidly cooling to prevent foaming, followed by grinding the resultant mixture; the decomposition being effected by heating said particles to a temperature capable of effecting melting of the metal to be foamed and thereafter effecting decomposition of said material adapted to effect foaming.

The metal foam may be cast in a mold and then removed therefrom after cooling to provide a metal foam body or may be formed in a hollow article or mold e.g. a hollow pipe to provide a hollow article having as an integral part therewithin a metal foam body.

This invention is adapted to foam any metal capable of being foamed. Metals which may be used include preferably aluminum, zinc, iron, lead, copper and nickel as well as alloys of these and other metals and such metals as magnesium and titanium may be suitable.

The metals preferred for the invention include those having a substantial temperature difference, preferably of 200 to 400 C., between the solid phase temperature and the liquid phase temperature. It is also helpful to utilize metal wherein its solid phase may be caused to predominate to cause decreased fluidity; the metal being foamed at the temperature at which this occurs, such temperature being between the solid and liquid phase temperature.

For foaming the metal, the addition of a foaming agent such as metal hydride particles of zirconium hydride or titanium hydride may be used. Other compounds may also be used under certain temperature and pressure conditions. Examples are ammonium chloride, ammonium iodide, ammonium sulfate, arsenic, barium hydride, bismuth sulfate, calcium carbonate, calcium hydride, calcium magnesium carbonate, calcium sulfite, ferric sulfate, lead carbonate, lead oxide and sodium nitrate.

The amount of hydride in the foaming agent which is necessary for foaming depends on the metal used, the density of the desired foam, the foaming agent used and also on whether the foaming agent is completely exhausted, i.e., whether all the gas capable of being liberated from said foamer has been liberated therefrom.

The foamer can be in the form of powder, granules or particles or in any form which can easily be dispersed throughout the metal to be foamed.

It is preferred that the foaming agent utilized decompose at a temperature slightly above the melting temperature of the metal to be foamed. In some cases, this may not be plausible and a foamer may be used having a decomposition temperature equal to or less than the melting point of the metal to be foamed. In the afore mentioned case, a pressure unit such as that described in Figure 2 may be utilized.

If a pressure unit is utilized, the pressure exerted on the mixture of metal and material adapted to effect foaming should be greater than the region into which the material is to be expelled. The pressure unit must be capable of preventing the escape of the initial gas added to the chamber as well as the escape of the vapor obtained by the decomposition of the foaming agent so that the vapor pressure can be built up within said unit so that the pressure within said unit exceeds the pressure outside the unit. The material upon being expelled from the unit gives a metal foam by expansion of the gas previously repressed from foaming by the pressure cxerted thereon. Any conventional apparatus may be employed so long as it is capable of withstanding the heat and pressure utilized.

Initial pressure in the pressure container may be obtained by the introduction of a gas such as oxygen, air or other oxygen-containing or inert gas or any gas which will neither react with the metal to be foamed nor have a deleterious effect on the metal to be foamed. The gas may be introduced into the pressure container by means adapted therefor and the amount of gas introduced may be controlled by the valve located on the gas container.

One reason for the application of pressure is to suppress foaming of the metal until all materials involved have attained the desired properties for foaming. Another reason is to allow the use of a foaming agent having a decomposition temperature lower than the desired decomposition temperature.

The amount of heat used must be great enough to allow melting of the metal to be foamed. The heat may be provided by any means capable of reaching the necessary temperature for the various metals to be foamed. For example, the heating may be obtained by gas or oil burners directed on the molten metal foam or by other suitable means. The heating zone may consist of burners above, below and on each side of the mixture of metal and foaming agent or may consist of any arrangement which will provide adequate heat for the invention.

The pressure within the container should be great enough to prevent the gas obtained by the decomposition of the foaming agent from expanding to form metal foam immediately or while within the container. When the mixture of metal and foaming agent is expelled from the pressure container, the gas formed by the decomposition of the foaming agent expands to form the desired metal foam.

If a mixture of metal particles, and particles of a material adapted to effect foaming is to be used, while preparing the mixture, a great deal of care must be taken to insure thorough mixing of said mixture so that particles of the foaming agent are uniformly dispersed throughout the metal to be foamed.

In some cases, it may be preferable to take hydridecontaining metal alloy, mix this material with the molten metal to be foamed which preferably is maintained relatively near its solidus temperature and then quickly cooled to suppress foaming the mixture. This material may be ground into particles and this mixture is then used in the same manner as the hydride-containing metal alloy and metal particle mixture.

The metal to be foamed can be of any shape and size although it is preferred that it is in the form of particles, powder, or granules and in the same form as is the foaming agent.

After formation of the metal foam, the foam preferably is cooled. This may be done by means of water, air, oil or other means capable of performing the desired duties.

The process may be carried out in a pressurized container, molds, casts or any other suitable container depending on the materials used and the desired shape of the finished product.

While the use of a vibrating hopper for the continuous process is preferred, we may also use other types of feeders; examples thereof including a screw-type feeder such as a helical screw or cone shaped container having a base which may be opened or closed, or a hopper having a mesh at the base thereof.

In a continuous process, we may continuously add a mixture of metal to be foamed and a material adapted to effect foaming preferably to a vibrating hopper, said hopper vibrating in such a manner to allow a steady stream of mixture to flow down a trough attached to the base of said hopper onto a conveyor belt. The conveyor belt moves through a heated zone, said zone being supplied with heat by gas burners, at a rate of speed which allows the metal of the mixture to melt and after melting of the metal causes the decomposition of the foaming agent. The molten metal foam while still on the conveyor belt, is then subjected to cooling in order to insure the rigidity of the walls of the discrete cellular bubbles within said metal foam.

If a foaming agent having a lower decomposition temperature than the melting temperature of the metal to be foamed is to be used, it is desirable to employ a pressurized container. If a pressure container is used for the continuous process, it must contain therein, a vibrating hopper or other feeder, a trough to carry the particles from said hopper down to the conveyor belt, a conveyor' belt, a heating zone for melting the metal to be foamed and a container through which the molten metal must travel before being expelled through an orifice onto another conveyor belt.

Before start-up of the process, the heating means of the heating zone are activated and the temperature within said container is brought to that temperature at which the metal to be foamed will melt. The particles to be foamed are added through an opening in the top of the pressure unit and the unit is then closed. The pressure is then built-up Within the container by the addition of a gas such as oxygen.

The process is then started by causing the hopper to vibrate thus freeing particles which move down the trough in a steady stream onto the conveyor belt thereunder. The conveyor belt gradually moves to expose the particles of said mixture to the action of suln'cient heat to melt the metal particles and to cause decomposition of the foaming agent; the foaming tendency of the foaming agent being repressed by the pressure built up within said container. The molten material then flows into a funnel-shaped container or other container and is expelled from the pressure container through an orifice and the release of the vapor pressure causes foaming or expansion of the gas contained within said molten metal. After expulsion through the orifice the molten metal foam flows onto another conveyor belt and the discrete cellular bubbles formed therewithin are made rigid by air cooling.

If a pressure unit is to be used, there are several variables which may be regulated. The vibrator may be regulated so that the mixture of metal and foaming agent is uniformly released. If the hopper release of the mixture is too slow, insufiicient foam may be produced unless the conveyor belts are slowed down and if the release is too rapid the amount of metal foam produced may be too great to be handled by the conveyor belt unless the belt is speeded up or the metal may roll off or the container leading to the orifice at the end of the conveyor may not be large enough to handle the amount of metal.

The speed of the conveyor belt may also be regulated. This may be done to obtain a faster process and also to overcome the difficulty of too rapid release of the mixture. The speed may be lessened too if the number of particles released on the belt produce too little foam. The conveyor belts may be preferably regulated so that the one on the outside of the pressure container runs at the same speed as the conveyor belt within the pressure container. If the belt within the container runs too rapidly, the orifice may be opened to prevent piling up of the foam therewithin and the outer belts speed may also be increased. If the belt outside of the conveyor runs too rapidly the metal foam may be made in insufficient amounts.

The size of the orifice through which the molten metal is expelled may be made variable and may be regulated. If the conveyor belt Within the pressure unit runs too fast the size of the orifice may then be increased to allow the molten metal to flow out onto the second belt and if too little foam is being produced the orifice may be narrowed to prevent escape of vapors.

If a batch type process is to be used, the vessel or mold may be preheated to a temperature at which the metal to be foamed melts. The particles may then be uniformly spread in a layer on the bottom of the vessel or mold so that the amount of metal upon foaming will completely fill the mold or vessel. After the mixture has been added to the mold or vessel, the mold or vessel may be exposed to heat capable of melting the metal to be foamed and thereafter capable of decomposing the material adapted to effect foaming; the heat being sup-- plied by means of gas burners, oil burners, or any means capable of supplying the necessary amount of heat.

The amount of heat depends directly on the melting point of the metal to be foamed. For example, if alumi num is to be foamed, a temperature above the melting point of the aluminum which is 658 C. is desired.

Having thus disclosed our invention, we claim:

1. The method for making a metal foam body comprising providing solid particles obtained as the mixing product of metal and a solid substance which decomposes and liberates gas when heated, then heating said solid particles to a temperature sufliciently high to pro vide molten metal and to decompose said solid substance to liberate said gas in the molten metal thus provided, thus forming cellular bubbles in the metal and then cooling the metal to provide a metal foam body.

2. The method of claim 1, wherein said particles comprise particles of said metal and particles of said substance.

3. The method of claim 1, wherein said particles com- 7 prise particles of a mixture of said metal and said substance.

4. The method of claim 1, wherein said particles of said substance comprise a hydride selected from the group consisting of titanium hydride and zirconium hydride.

5. The method of claim 1, wherein said metal has a liquid phase temperature at least 100 C. higher than its solid phase temperature and said metal is maintained at a temperature between said solid phase temperature and said liquid phase temperature to provide substantially reduced fluidity.

6. The method for making a metal foam body comprising providing a solid body obtained as the mixing product of metal and a solid substance which decomposes and liberates gas when heated, then heating said solid body to a temperature sufficiently high to provide molten metal and to decompose said solid substance to provide cellular bubbles of said gas in the molten metal thus provided and then cooling to provide said metal foam body.

7. The method of claim 6 wherein said substance comprises a hydride selected from the group consisting of titanium hydride and zirconium hydride.

8. The method of claim 6 wherein said solid body is a particle among a large number of other like particles.

9. The method of claim 8 wherein said particles comprise particles of said metal and particles of said substance.

10. The method of claim 8, wherein said particles comprise particles of a mixture of said metal and said sub stance.

11. The method of claim 8 wherein said particles of said substance comprise a hydride selected from the group consisting of titanium hydride and zirconium hydride.

12. The method for making a metal foam body comprising providing solid particles obtained as the mixing product of metal and a solid substance which decomposes and liberates gas when heated, then while providing for unrestrained expansion in volume of said solid particles, heating said solid particles to a temperature sufliciently high to provide molten metal and to decompose said solid substance to liberate said gas in the molten metal thus provided, thus forming cellular bubbles in the metal and then cooling the metal to provide a metal foam body.

13. The method of claim 12, wherein said particles comprise particles of said metal and particles of said substance.

14. The method of claim 12 wherein said particles comprise particles of a mixture of said metal and said substance.

15. The method of claim 12, wherein said particles of said substance comprises a hydride selected from the group consisting of titanium hydride and zirconium hydride.

16. The method for making a metal foam body comprising providing a solid body obtained as the mixing product of metal and a solid substance which decomposes and liberates gas when heated, then heating said solid body while under superatmospheric gas pressure to a temperature sufficiently high to provide molten metal and to decompose said solid substance, then releasing said pressure to provide cellular bubbles of said gas in the molten metal thus provided and then cooling to provide said metal foam body.

17. The method of claim 16 wherein said substance comprises a hydride selected from the group consisting of titanium hydride and zirconium hydride.

18. The method of claim 16 wherein said solid body is a particle among a large number of other like particles.

19. The method of claim 16, wherein said particles comprise particles of a metal and particles of a substance which decomposes and liberates gas when heated.

20. The method of claim 16, wherein said particles comprise particles of a mixture of a metal and a substance which decomposes and liberates gas when heated.

21. The method of claim 16 wherein said particles of said substance comprise a hydride selected from the group consisting of titanium hydride and zirconium hydride.

References Cited in the file of this patent UNITED STATES PATENTS 1,612,737 Lane Dec. 28, 1926 1,850,743 Connelly et al. Mar. 22, 1932 2,303,679 Brown Dec. 1, 1942 2,395,291 Patterson Feb. 19, 1946 2,434,775 Sosnick Jan. 20, 1948 2,517,223 Mantell Aug. 1, 1950 2,553,016 Sosnick May 15, 1951 2,751,289 Elliott June 19, 1956 2,787,816 Brennan Apr. 9, 1957 FOREIGN PATENTS 615,147 France Dec. 30, 1926 729,339 Great Britain May 4, 1955 

1. THE METHOD FOR MAKING A METAL FOAM BODY COMPRISING PROVIDING SOLID PARTICLES OBTAINED AS THE MIXING PRODUCT OF METAL AND A SOLID SUBSTANCE WHICH DECOMPOSES AND LIBERATES GAS WHEN HEATED, THEN HEATING SAID SOLID PARTICLES TO A TEMPERATURE SUFFICIENTLY HIGH TO PROVIDE MOLTEN METAL AND TO DECOMPOSE SAID SOLID SUBSTANCE TO LIBERATE SAID GAS IN THE MOLTEN METAL THUS PROVIDED, THUS FORMING CELLULAR BUBBLES IN THE METAL AND THEN COOLING THE METAL TO PROVIDE A METAL FOAM BODY. 